Method of preventing polymer scale deposition and polymer scale preventive agent

ABSTRACT

A method of preventing polymer scale deposition during polymerization of a monomer having an ethylenically double bond in a polymerization vessel, comprising the step of carrying out said polymerization in a polymerization vessel of which the inner wall surface has a coating comprising: 
     (A) a polyvinyl alcohol, and 
     (B) a water-soluble polymeric compound having a carboxyl group which may be in the form of a salt, and a polymer scale preventive agent comprising said (A) and (B). Deposition of polymer scale can be effectively prevented. Since the components (A) and (B) have no poisonousness, safety of operators are ensured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of preventing polymer scale deposition on the inner wall surface of a polymerization vessel during polymerization of a monomer having an ethylenically double bond.

2. Description of the Prior Art

In processes of preparing polymers by polymerizing a monomer in a polymerization vessel, the problem that polymer deposits on the inner wall surface in the form of scale, is known.

The deposition of the polymer scale on the inner wall results in disadvantages that the yield of the polymer and the cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into manufactured polymer, thereby impairing the quality of the manufactured polymer; and that removal of such polymer scale is laborious and hence time-consuming. Further, since the polymer scale contains unreacted monomer and operators may be exposed thereto, which may cause physical disorders in the operators.

Heretofore, as methods for preventing polymer scale deposition on the inner wall surface and so forth of a polymerization vessel, methods in which a suitable substance is coated on the inner wall surface, etc. as a scale preventive agent.

As the suitable substance acting as a scale preventive agent, there has been disclosed, for example, particular polar compounds (Japanese Patent Publication (KOKOKU) No.30343/1970), a dye or pigment (Japanese Patent Publication (KOKOKU) Nos. 30835/1970, and 24953/1977), etc.

However, many of the prior art polymer scale preventive agents comprised of the substances described above are poisonous as represented by aniline, nitrobenzene, formaldehyde, etc. listed as examples of the polar organic compounds described in Japanese Patent Publication (KOKOKU) No. 30343/1970, and the pigments containing a heavy metal such as chromium and lead described in Japanese Patent Publication (KOKOKU) No. 30835/1970. The dyes described in Japanese Patent Publication (KOKOKU) Nos. 30835/1970 and 24953/1977 include some dyes that involve concern about carcinogenesis. Therefore, use of these substances may cause problems in safety of operators.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a polymer scale preventive agent which are capable of preventing polymer scale deposition effectively, and are not poisonous and therefore causes no problem with respect to safety of operators, and a method of preventing polymer scale using the same.

Thus, the present invention provides, as a means of achieving said object, a method of preventing polymer scale deposition during polymerization of a monomer having an ethylenically double bond in a polymerization vessel, comprising the step of carrying out said polymerization in a polymerization vessel of which the inner wall surface has a coating comprising:

(A) a polyvinyl alcohol, and

(B) a water-soluble polymeric compound having a carboxyl group which may be in the form of a salt.

The present invention also provides a polymer scale preventive agent for use in polymerization of a monomer having an ethylenically double bond, comprising:

(A) a polyvinyl alcohol, and

(B) a water-soluble polymeric compound having a carboxyl group which may be in the form of a salt.

According to the present invention, deposition of polymer scale can be effectively prevented; since the (A) polyvinyl alcohol and the (B) water-soluble polymeric compound having a carboxyl group used as components of the coating are not poisonousness, and safety of operators are ensured.

Also, according to the present invention, polymer scale deposition can be effectively prevented, irrespectively of polymerization conditions such as the kinds of a monomer and a polymerization initiator, polymerization form, the kind of the material constituting the inner wall of polymerization vessels, etc. That is, deposition of polymer scale can be effectively prevented even in emulsion polymerization in which polymer scale deposition has been so far difficult to prevent, in the case where a polymerization vessel made of stainless steel in which polymer scale has been so far liable to deposit is used, or in the case where a polymerization initiator with a strong oxidizing action such as potassium peroxodisulfate and the like is used.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS (A) Polyvinyl alcohol

The polyvinyl alcohol as the component (A) of the coating is not poisonousness, and is suitable to the object of the present invention.

The polyvinyl alcohol includes, for example, completely saponified products of polyvinyl acetates and partially saponified products thereof. The polyvinyl alcohol normally has a backbone chain consisting of repeated vinyl alcohol units of 1,3-glycol type, but may contain from 1 to 2 mol % of units of 1,2-glycol type; about 10 to 20% of the terminals of the molecular chains may have contain a terminal carboxyl group; and the polyvinyl alcohol may contain conjugated carbonyl groups. These polyvinyl alcohols can be used singly or in combination of two or more.

Among the polyvinyl alcohols, particularly preferred with respect to polymer scale preventing effect are those having a viscosity in the form of 4% by weight aqueous solution at 20° C. of from 3 to 100 cP, and a saponification degree in the range of from 50 to 100 mol %.

In the meantime, vinyl chloride polymers obtained by polymerization are required to have a high whiteness. For example, it is required to have a lightness index L in the Hunter's color difference equation described in JIS Z 8730 (1980) of 70 or more.

On the other hand, many of the prior art polymer scale preventive agents are colored substances as exemplified typically by the dyes and pigments described in Japanese Patent Publication (KOKOKU) No. 30835/1970 or 24953/1977, the aromatic amine compounds described in Japanese Pre-examination Patent Publication (KOKAI) No. 50887/1976, and the reaction products of a phenolic compound and an aromatic aldehyde described in Japanese Preexamination Patent Publication (KOKAI) No. 54317/1980, etc. Due to this, if a coating composed of such a colored polymer scale preventive agent is dissolved or peels, and components thereof are incorporated into the polymerization system, product polymer obtained may be colored.

However, the above polyvinyl alcohols per se used as the component (A) in the present invention, are generally colorless. In addition, the coating formed dissolves in a polymerization system or peel, with difficulty. The compounds are hardly mixed into manufactured polymers. Hence, appropriate selection of the component B) described below makes it possible to prevent coloration of polymers and to produce polymers having a high whiteness with an L value of 70 or more.

(B) Water-soluble polymeric compound having a carboxyl group

The water-soluble polymeric compound having a carboxyl group as the component (B) of the coating is not poisonousness; hence it is suitable to the object of the present invention.

Such water-soluble polymeric compounds having a carboxyl group include various examples. Among them, the following examples are preferred in that they are not poisonous as well as they are suited to production of polymers with high whiteness.

Specifically, preferred are, for example, polyacrylic acids, polymethacrylic acids, acrylic acid/methacrylic acid copolymers, maleic anhydride/vinyl ether copolymers, isobutylene/maleic anhydride copolymers, styrene/maleic anhydride copolymers, vinyl acetate/maleic anhydride copolymers, and alkali metal salts such as sodium salts and potassium salts or ammonium salts of these polymers. These can be used singly or in combination of two or more; the combination is selected suitably.

Among the components (B), particularly preferred with respect to polymer scale preventing effect are acrylic acid polymers such as polyacrylic acid, polymethacrylic acid, and acrylic acid/methacrylic acid copolymers.

Formation of coating

The coating comprising said component (A) and component (B) can be formed, for example, by applying a coating solution containing these components to the inner wall surface of a polymerization vessel and then drying.

The coating solution can be prepared by dissolving or dispersing the component (A) and the component (B) in a solvent described later.

The total concentration of the component (A) and the component (B) is not particularly limited, as long as a desired coating weight, i.e., total weight of the component (A) and component (B) per a unit area, described later is obtained. It may be normally from about 0.005 to 20% by weight, preferably from about 0.01 to 10% by weight. In the coating formed, the component (B) is normally contained in an amount of from 0.1 to 1,000 parts by weight, preferably from 1 to 600 parts by weight, per 100 parts by weight of the component (A). Also, in the coating solution, the components (A) and (B) may normally contained in the same amounts as above. If the content of the component (B) is too large or too small relatively to the component (A), the scale preventing effect due to the combined use of the component (A) and the component (B) may not be achieved.

The solvent used for preparation of the coating solution include, for example, water; alcohols such as methanol, ethanol, propanol, butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 3-methyl-1-butanol, 2-methyl-2-butanol, and 2-pentanol; ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; esters such as methyl formate, ethyl formate, methyl acetate, and methyl acetoacetate; ethers such as 4-methyl dioxolan, and ethylene glycol diethyl ether; furans; and aprotic solvents such as dimethylformamide, dimethyl sulfoxide, and acetonitrile. These solvents are appropriately used singly or as a mixed solvent of two or more.

Moreover, to the polymer scale preventive agent of the present invention, for example, a cationic, nonionic, anionic surfactant or the like can be added as long as it impairs the polymer scale preventing action. An inorganic compound is also optionally added, as long as it impairs neither the polymer scale preventing action nor the non-poisonousness which are intended by the present invention, preferably, in such an amount not as to impair the whiteness of manufactured polymers.

The inorganic compound which may be added to the polymer scale preventive agent of the present invention includes, for example, silicic acids and silicates such as orthosilicic acid, metasilicic acid, mesosilicic acid, mesotrisilicic acid, mesotetrasilicic acid, sodium metasilicate, sodium orthosilicate, sodium disilicate, sodium tetrasilicate, and water glass; metallic compounds such as oxygen acid salts, acetates, nitrates, hydroxides or halides of a metal selected from the group consisting of alkali earth metal elements such as calcium, zinc family elements such as zinc, aluminum family elements such as aluminum, and platinum family elements such as platinum; and inorganic colloids such as ferric hydroxide colloid, colloid of silicic acid, colloid of barium sulfate, and colloid of aluminum hydroxide. These inorganic colloids can be prepared, for example, by mechanical crushing, irradiation with ultrasonic wave, electrical dispersion or chemical methods.

The coating solution thus prepared can be applied to any portions where polymer scale may deposit, so as to form the coating at the portions, irrespectively of the material of the portions; thereby deposition of polymer scale can be prevented at the portions.

Formation of the coating on the inner wall surface of a polymerization vessel using the coating solution above, is conducted by first applying the coating solution to the inner wall surface of the polymerization vessel, then drying the applied coating solution sufficiently, for example, at a temperature in the range of from room temperature to 100° C., if necessary, followed by washing with water. The coating thus formed on the inner wall surface prevents polymer scale from depositing on the inner wall surface during polymerization.

The method of applying the coating solution is not particularly limited, and includes, for example, the brush coating, spray coating, the method of filing the polymerization vessel with the coating solution followed by withdrawal thereof, and automatic coating methods as disclosed in Japanese Pre-examination Patent Publication (KOKAI) Nos. 61001/1982 and 36288/1980, and Japanese Preexamination Patent Publication (KOHYO) Nos. 501116/1981 and 501117/1981, and Japanese Pre-examination Publication (KOKAI) Nos. 11303/1984, etc.

The method of drying wet coated surfaces provided by application of the coating solution, is not limited, either. Following methods can be used. That is, a method in which, after the solution is applied, hot air with an suitable elevated temperature is blown to the coated surface, and a method in which the inner wall surface of a polymerization vessel and the surfaces of other parts to be coated are previously heated to from 30° to 80° C., and the coating solution is directly applied to the heated inner wall surfaces, etc. After dried, the coated surfaces are washed with water if necessary.

Said coating solution is preferably applied to not only the inner wall surface of a polymerization vessel but also other parts with which the monomer comes into contact during polymerization to form the coating on such parts. For example, it is preferred to apply the coating solution to a stirring shaft, stirring blades, condensers, headers, search coil, bolts, nuts, etc. to form the coating thereon. Particularly, on stirring blades, stirring shaft and baffles, the coating should be formed by application of the coating solution. The formation of the coating on these parts by application of the coating solution can be performed in the same manner as in the formation of the coating on the inner wall surface.

More preferably, for formation of the coating, said coating solution is applied to portions with which monomer does not come into contact during polymerization but on which polymer scale may deposit, for example, portions of recovery system for unreacted monomer with which unreacted monomer comes into contact, such as inner surfaces, etc. of equipment and pipes of the recovery system. Specifically, such portions include the inner surfaces of monomer distillation columns, condensers, monomer stock tanks and valves. Formation of the coating on such portions also may be performed in the same manner as the formation of the coating on the inner wall surface of a polymerization vessel of a polymerization vessel.

Where the coating is formed as described above on the portion with which monomer comes into contact during polymerization and other portions where polymer scale may deposit, deposition of polymer scale is prevented on these portions.

The coating formed as described above has normally a coating weight, i.e., a total weight of the component (A) and the component (B) per a unit area, of 0.001 g/m² or more, preferably from 0.05 to 2 g/m², after dried.

The coating operation described above may be conducted every 1 to ten-odd batches of polymerization. The formed coating has good durability and retains the scale-preventing action; therefore the coating operation is not necessarily performed every batch of polymerization. Consequently, productivity in production of polymer is improved.

Furthermore, said coating solution used for the coating operation can be used not only for the coating operation on the inner wall surface, etc. of a polymerization vessel, but also for addition in a small amount to polymerization medium. Such addition of the coating solution enhances the polymer scale preventing effect. The amount of the coating solution added to the polymerization medium normally may range from 5 to 1,000 ppm based on the whole weight of the monomer charged.

Polymerization

After the formation of the coating on the inner wall surface of a polymerization vessel, other parts with which monomer may come into contact, etc. by coating operation, polymerization is carried out in accordance with conventional procedures. That is, a monomer having an ethylenically double bond, a polymerization initiator (catalyst), and optionally a polymerization medium such as water, etc., a suspending agent, a solid dispersing agent, a dispersing agent such as nonionic or anionic surfactants are charged into the polymerization vessel, followed by carrying out polymerization according to conventional methods.

The monomer having an ethylenically double bond to which the method of this invention can be applied may include, for example, vinyl halides such as vinyl chloride; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid, methacrylic acid, and esters or salts of these; maleic acid, fumaric acid, and esters or anhydrides thereof; and diene monomers such as butadiene, chloroprene and isoprene; as well as styrene, acrylonitrile, halogenated vinylidenes, and vinyl ethers.

There are no particular limitations on the form of polymerization to which the method of this invention can be applied. The present invention is effective in any forms of polymerization such as suspension polymerization, emulsion polymerization, solution polymerization, bulk polymerization, and gas phase polymerization. Particularly, the present method is more suitable to polymerizations in an aqueous medium such as suspension or emulsion polymerization.

In the following, general polymerization conditions are described on each form of polymerization.

In the case of suspension or emulsion polymerization, first, water and a dispersing agent are charged into a polymerization vessel, and thereafter a polymerization initiator is charged. Subsequently, the inside of the polymerization vessel is evacuated to a pressure of from 0.1 to 760 mmHg, and a monomer is then charged (whereupon the pressure inside the polymerization vessel usually becomes from 0.5 to 30 kgf/cm².G). Thereafter, polymerization is carried out at a temperature of from 30° to 150°During the polymerization, one or more of water, a dispersing agent and a polymerization initiator may be added, if necessary. Reaction temperature during the polymerization is different depending on the kind of monomer to be polymerized. For example, in the case of polymerizing vinyl chloride, polymerization is carried out at 30° to 80° C.; in the case of polymerizing styrene, polymerization is carried out at 50° to 150° C. The polymerization may be judged to be completed when the pressure inside the polymerization vessel falls to from 0 to 7 kgf/cm².G or when cooling water which passes through a jacket provided around the polymerization vessel indicates almost the same temperature at the inlet where it is charged and at the outlet where it is discharged (i.e., when exothermic heat due to polymerization reaction becomes zero). The water, dispersing agent and polymerization initiator to be charged for polymerization are used in amounts of 20 to 500 parts by weight, 0.01 to 30 parts by weight, and 0.01 to 5 parts by weight, respectively, per 100 parts by weight of the monomer.

In the case of solution polymerization, an organic solvent such as toluene, xylene and pyridine is used as the polymerization medium in place of water. The dispersing agent is optionally used. The other conditions are generally the same as those described for suspension and emulsion polymerizations.

In the case of bulk polymerization, after the inside of a polymerization vessel is evacuated to a pressure of from about 0.01 mmHg to about 760 mmHg, a monomer and a polymerization initiator are charged into the polymerization vessel, and then polymerization is carried out at a temperature of from -10° C. to 250° C. Specific methods of the bulk polymerization includes, for example, liquid bulk polymerization and gas phase polymerization for vinyl chloride.

Application of the method of preventing polymer scale deposition of the present invention to polymerization, makes it possible to prevent polymer scale from depositing regardless of materials constituting the inner wall, etc. of a polymerization vessel. For example, this method can prevent deposition of polymer scale even in the case polymerization is carried out in a polymerization vessel made of a steel including stainless steel or lined with glass.

Any additive materials that have been conventionally added in a polymerization system can be used without any limitation. More specifically, the method of this invention can effectively prevent polymers from depositing, even in polymerization systems containing the following additives: for example, polymerization catalysts such as t-butyl peroxyneodecanoate, bis(2-ethylhexyl) peroxydicarbonate, 3,5,5-trimethylhexanoyl peroxide, α-cumyl peroxydicarbonate, cumene hydroperoxide, cyclohexanone peroxide, t-butyl peroxypivalate, bis(2-ethylhexyl) peroxydicarbonate, benzoyl peroxide, lauroyl peroxide, 2,4-dichlorobenzoyl peroxide, diisopropyl peroxydicarbonate, α,α'-azobisisobutyronitrile, α,α'-azobis-2,4-dimethylvaleronitrile, potassium peroxodisulfate, ammonium peroxodisulfate, p-menthane hydroperoxide; suspension agents comprised of natural or synthetic polymeric compounds such as partially saponified polyvinyl alcohols, polyacrylic acids, vinyl acetate/maleic anhydride copolymer, cellulose derivatives such as hydroxypropylmethyl cellulose, and gelatin; solid dispersing agents such as calcium phosphate and hydroxyapatite; nonionic emulsifying agents such as sorbitan monolaurate, sorbitan trioleate and polyoxyethylene alkyl ether; anionic emulsifying agents such as sodium lauryl sulfate, sodium alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate and sodium dioctylsulfosuccinate; fillers such as calcium carbonate and titanium oxide; stabilizers such as tribasic lead sulfate, calcium stearate, dibutyltin dilaurate and dioctyltin mercaptide; lubricants such as rice wax, stearic acid and cetyl alcohol; plasticizers such as DOP and DBP; chain transfer agents such as mercaptans such as t-dodecyl mercaptan, and trichloroethylene; and pH adjusters.

EXAMPLES

The present invention is now described in detail by way of working examples and comparative examples. In each of the tables below, Experiment Nos. marked with * indicate comparative examples, and the other Experiment Nos., working examples of the present invention.

Example 1

Polymerization was carried out in the following manner using a polymerization vessel with an inner capacity of 1,000 liters and having a stirrer.

In each experiment, first, a component (A) and a component (B) were dissolved in a solvent so that the total concentration thereof might become the value given in Table 1, to prepare a coating solution. The coating solution was applied to the inner wall and other parts with which a monomer comes into contact during polymerization, including the stirring shaft and stirring blades, followed by drying under heating at 60° C. for 15 min. to form a coating, which was then washed with water.

However, Experiment Nos. 101 to 103 are comparative examples in which no coating solution was applied, or a coating solution containing either a component (A) or a component (B) was applied; Experiment No.110 is another comparative example in which the coating solution used in experiment No. 105 was not applied but was added to the polymerization medium; Experiment No. 114 is a comparative example in which a coating solution containing Sudan B, Nigrosine and Na₂ SiO₃ (weight ratio: 100/100/100) in methanol (which is one used in Experiment No. 33 described in Japanese Patent Publication (KOKOKU) No. 24953/1977), was used.

The kind of the (A) polyvinyl alcohol and the (B) acrylic acid polymer used, the total concentration of (A)+(B) and the weight ratio of (A)/(B) in the coating solution, and the solvent used in each experiment are given in Table 1.

Subsequently, in the polymerization vessel in which a coating had been formed by the coating operation above, were charged 400 kg of water, 200 kg of vinyl chloride, 250 g of partially saponified polyvinyl alcohol, 25 g of hydroxypropylmethyl cellulose and 75 g of bis(2-ethylhexyl) peroxydicarbonate. In addition, in Experiment Nos. 108-110, the coating solution of Experiment No. 105 in the amount as given in Table 1 was added to the polymerization medium. Then, polymerization was carried out at 57° C. with stirring for 6 hours. After the completion of the polymerization, the amount of polymer scale depositing on the inner wall of the polymerization vessel (Amount of polymer scale deposition) was measured. Further, the whiteness of the polymer obtained in each Experiment was measured according to the following method.

A hundred parts by weight of a polymer, one part by weight of a dibutyl tin laurate stabilizing agent TS-101 (tradename, product of Akishima Chemical Co.) and 0.5 part by weight of a cadmium complex-type liquid stabilizing agent C-100J (tradename, product of Katsuta Kako Co.), and 50 parts by weight of a plasticizer DOP were kneaded at 160° C. for 5 min. with a twin roll mill, and then formed into a sheet 1 mm thick. Subsequently, this sheet was placed in a mold measuring 4 cm×4 cm×1.5 cm (thickness), and molded under heating at 160° C. and under the pressure of 65 to 70 kgf/cm² to prepare a test specimen. This test specimen was measured for lightness L in the Hunter's color difference equation described in JIS Z 8730 (1980). Whiteness was evaluated to become higher with increase in L value.

The L value was determined as follows.

The stimulus value Y of XYZ color system was determined according to the photoelectric tristimulus colorimetry using the standard light C, a photoelectric colorimeter (Color measuring color difference meter Model Z-1001DP, product of Nippon Denshoku Kogyo K.K.) in accordance with JIS Z 8722. As the geometric condition for illumination and receiving of light, the condition (d) defined in section 4.3.1 of JIS Z 8722 was adopted. Next, L was calculated based on the equation: L=10Y^(1/2) described in JIS Z 8730(1980).

The results are given in Table 1.

It is noted that in the columns of (A) polyvinyl alcohol and (B) acrylic acid polymer in Table 1, the tradenames of the polymers used are given in parentheses. Regarding the polyvinyl alcohols used, viscosity, saponification degree and manufacturer are given in Table 2; regarding the acrylic acid polymers, solid content, viscosity and manufacturer are given in Table 3.

                                      TABLE 1                                      __________________________________________________________________________     Coating solution                              Amount *1                                                                            Amount of                                         (A) + (B) total                                                                        Weight         added to                                                                             polymer                    Exp.          (B) Acrylic                                                                             concentration                                                                          ratio Solvent  medium                                                                               scale                      No.                                                                               (A) Polyvinyl alcohol                                                                     acid polymer                                                                            (wt. %) (A)/(B)                                                                              (weight ratio)                                                                          (ppm) (g/m.sup.2)                                                                          L                    __________________________________________________________________________                                                               value                101*                                                                               --         --      --      --     --      --    1300  73                   102*                                                                              Saponified polyvinyl                                                                       --      0.5     100/0 Water/Methanol                                                                          --    1100  73                      alchohol (KH-20)                  (90/10)                                   103*                                                                               --        Polyacrylic acid                                                                        0.5      0/100                                                                               Water/Methanol                                                                          --    1150  72.5                               (AC-10H)               (90/10)                                   104                                                                               Saponified polyvinyl                                                                      Polyacrylic acid                                                                        0.5     100/10                                                                               Water/Methanol                                                                          --    10    72.5                    alcohol (KH-20)                                                                           (AC-10H)               (90/10)                                   105                                                                               Saponified polyvinyl                                                                      Polyacrylic acid                                                                        0.5     100/100                                                                              Water/Methanol                                                                          --    9     73                      alcohol (KH-20)                                                                           (AC-10H)               (90/10)                                   106                                                                               Saponified polyvinyl                                                                      Polyacrylic acid                                                                        0.5     100/500                                                                              Water/Methanol                                                                          --    6     72.5                    alcohol (KH-20)                                                                           (AC-10H)               (90/10)                                   107                                                                               Saponified polyvinyl                                                                      Polyacrylic acid                                                                        0.5      100/1000                                                                            Water/Methanol                                                                          --    90    72                      alcohol (KH-20)                                                                           (AC-10H)               (90/10)                                   108                                                                               Saponified polyvinyl                                                                      Polyacrylic acid                                                                        0.5     100/100                                                                              Water/Methanol                                                                           50   1     72                      alcohol (KH-20)                                                                           (AC-10H)               (90/10)                                   108                                                                               Saponified polyvinyl                                                                      Polyacrylic acid                                                                        0.5     100/100                                                                              Water/Methanol                                                                          500   0     71                      alcohol (KH-20)                                                                           (AC-10H)               (90/10)                                   110*                                                                               --         --      --      --     --      500   900   72                   111                                                                               Polyvinyl alcohol                                                                         Polyacrylic acid                                                                         0.05   100/300                                                                              Water    --    8     72                      (NH-20)    (AC-10S)                                                         112                                                                               Polyvinyl alcohol                                                                         Polymethacrylic                                                                          0.05   100/400                                                                              Water    --    8     73                      (NL-05)    acid (AC-30H)                                                    113                                                                               Saponified polyvinyl                                                                      Polyacrylic acid                                                                         0.05   100/50                                                                               Water/Methanol                                                                          --    6     71                      alcohol (L-9)                                                                             (AC-10SH)              (70/30)                                   114*                                                                              Coating solution of Exp. No. 33 described in Japanese Patent                   Publication (KOUKOKU) No. 24953/1977                                        __________________________________________________________________________      Remarks                                                                        *1: Coating solution of Exp. No. 105 was added.                          

                  TABLE 2                                                          ______________________________________                                         polyvinyl alcohols                                                             Viscosity                                                                      a 4% aqueous                                                                   solution      Saponification                                                   (cP, 20° C.)                                                                          degree     Manufacturer                                          ______________________________________                                         KH-20  48 ± 4  78.5-81.5  Nippon Synthetic                                                               Chemical Industry Co.                             NH-20  39 ± 4  99.0-100   Nippon Synthetic                                                               Chemical Industry Co.                             NL-05   5.3 ± 0.7                                                                             98.5-100   Nippon Synthetic                                                               Chemical Industry Co.                             L-9      6-6.5    70.0-72.0  KURARAY Co.                                       ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                         Acrylic acid polymers                                                          Solid        Viscosity                                                         content      (cP, 25° C.)                                                                        Manufacturer                                          ______________________________________                                         AC-10H 20 ± 1 20,000-40,000                                                                              Nippon Junyaku K.K.                               AC-10S 40 ± 1 100-300      "                                                AC-30H 19-21      80,000-130,000                                                                             "                                                AC-10SH                                                                                9-11      70,000-200,000                                                                             "                                                ______________________________________                                    

EXAMPLE 2

In each experiment, the coating procedure of Example 1 was repeated for a stainless steel polymerization vessel with an inner capacity of 20 liters and equipped with a stirrer, except for using the coating solution of which the (A) polyvinyl alcohol, (B) acrylic polymer and solvent, the total concentration of (A) +(B), and the weight ratio of (A)/(B) are as shown in Table 4. However, Experiment Nos. 201-203 are comparative examples in which no coating solution was coated, or a coating solution containing either a component (A) or a component (B) was used.

Subsequently, in the polymerization vessel thus coating-treated, were charged 9 kg of water, 225 g of sodium dodecylbenzenesulfonate, 12 g of t-dodecyl mercaptan, and 13 g of potassium peroxodisulfate. After the inner atmosphere was replaced with a nitrogen gas, 1.3 kg of styrene and 3.8 kg of butadiene were charged, followed by polymerization at 50° C. for 20 hours.

After completion of the polymerization, the amount of polymer scale depositing on the inner wall of the polymerization vessel was measured. The results are given in Table 4.

                                      TABLE 4                                      __________________________________________________________________________     Coating solution                             Amount of                                                (A) + (B) total                                                                        Weight        polymer                                          (B) Acrylic                                                                            concentration                                                                          ratio                                                                               Solvent  scale                             No. (A) Polyvinyl alcohol                                                                     acid polymer                                                                           (wt. %) (A)/(B)                                                                             (weight ratio)                                                                          (g/m.sup.2)                       __________________________________________________________________________     201*                                                                                --         --     --      --    --      400                               202*                                                                               Saponified polyvinyl                                                                       --     0.2     100/0                                                                               Water/Methanol                                                                          390                                   alcohol (L-9)                   (90/10)                                    203*                                                                                --        Polyacrylic acid                                                                       0.2      0/100                                                                              Water/Methanol                                                                          390                                              (Ac-10SH)            (90/10)                                    204 Saponified polyvinyl                                                                      Polycarylic acid                                                                       0.2     100/400                                                                             Water/Methanol                                                                           15                                   alcohol (L-9)                                                                             (AC-10SH)            (90/10)                                    205 Polyvinyl alcohol                                                                         Polyacrylic acid                                                                       1.0     100/100                                                                             Water     18                                   (NH-20)    (AC-10H)                                                        206 Polyvinyl alcohol                                                                         Polyacrylic acid                                                                       1.0     100/200                                                                             Water     16                                   (NH-20)    (AC-10H)                                                        __________________________________________________________________________ 

We claim:
 1. A method of preventing polymer scale deposition during polymerization of a monomer having an ethylenically double bond in a polymerization vessel, comprising the step of carrying out said polymerization in a polymerization vessel of which the inner wall surface has a coating comprising:(A) a polyvinyl; alcohol, and (B) a water-soluble polymeric compound having a carboxyl group which may be in the form of a salt, wherein said (B) water-soluble polymeric compound having a carboxyl group comprises at least one member selected from the group consisting of polyacrylic acids, polymethacrylic acids, acrylic acid/methacrylic acid copolymers, maleic anhydride/vinyl ether copolymers, isobutylene/maleic anhydride copolymers, styrene/maleic anhydride copolymers, vinyl acetate/maleic anhydride copolymers, and alkali metal salts and ammonium salts of these polymers, and wherein said coating contains said component (B) in an amount of from 0.1 to 1,000 parts by weight per 100 parts by weight of said component (A).
 2. The method according to claim 1, wherein said (A) polyvinyl alcohol has a saponification degree in the range of from 50 to 100 mol % and a viscosity in the form of 4% by weight aqueous solution of from 3 to 100 cP at 20° C.
 3. The method according to claim 1, wherein said (B) water soluble polymeric compound comprises at least one member selected from the group consisting of polyacrylic acids, polymethacrylic acids, and acrylic acid/methacrylic acid copolymers.
 4. The methods according to claim 1, wherein said coating has a coating weight of 0.001 g/m² or more.
 5. The method according to claim 1, wherein said coating is formed by applying a coating solution containing said components (A) and (B) to the inner wall surface of the polymerization vessel, followed by drying.
 6. The method according to claim 1, wherein said coating solution contains said component (B) in an amount of from 0.1 to 1,000 parts by weight per 100 parts by weight of said component (A), and has a total concentration of said components (A) and (B) of from 0.005 to 20% by weight.
 7. The method according to claim 1, wherein said coating has been also previously formed on parts of the polymerization vessel with which the monomer comes into contact during polymerization other than its inner wall surface.
 8. The method according to claim 7, wherein said parts with which the monomer comes into contact comprise at least one member selected from the group consisting of a stirring shaft, stirring blades, baffles, headers, search coils and condensers.
 9. The method according to claim 1, wherein said coating has been further previously formed at parts of the recovery system for unreacted monomer with which unreacted monomer comes into contact.
 10. The method according to claim 1, wherein the monomer is at least one member selected from the group consisting of vinyl halides; vinyl esters; acrylic acid, methacrylic acid, and esters and salts thereof; maleic acid and fumaric acid, and esters and anhydrides thereof; diene monomers; aromatic vinyl compounds; acrylates; acrylonitrile; halogenated vinylidenes; and vinyl ethers.
 11. The method according to claim 1, wherein said polymerization is carried out as suspension polymerization, emulsion polymerization, solution polymerization, bulk polymerization, or gas phase polymerization.
 12. The method of claim 1, wherein said coating solution is further added to the polymerization medium.
 13. A polymerization vessel of which the inner wall has a coating comprising:(A) a polyvinyl alcohol, and (B) a water-soluble polymeric compound having a carboxyl group which may be in the form of a salt, wherein said (B) water-soluble polymeric compound having a carboxyl group comprises at least one member selected from the group consisting of polyacrylic acids, polymethacrylic acids, acrylic acid/methacrylic acid copolymers, maleic anhydride/vinyl ether copolymers, isobutylene/maleic anhydride copolymers, styrene/maleic anhydride copolymers, vinyl acetate/maleic anhydride copolymers, and alkali metal salts and ammonium salts of these polymers, and wherein said coating contains said component (B) in an amount of from 0.01 to 1,000 parts by weight per 100 parts by weight of said component (A). 